The present invention relates to a spacer for coaxial tube systems having concentric tubes which have different temperatures such as occur in a cryogenic conduit and/or cable system, or a conduit system for hot fluids.
Spacers are known for coaxial tube systems which are of helical configuration and provide for some but very limited contact with the tubes. Such spacers are, for example, constructed from multiple ply layer systems. See e.g. U.S. Pat. Nos. 3,592,238 and 3,670,772. The tapes or ribbons used here have a rather low thermal conductivity and engage each other at isolated spots and areas. Also, the bundle is twisted to minimize contact with the tubes further.
A cryogenic conduit system has, for example, an inner tube which contains the conductors and/or is otherwise filled with and passed through by liquid helium. This tube is inserted in plural outer tubes which are provided for establishing a controlled temperature gradient. For example, the ring space between the inner tube and the next one may be evacuated, the space between the latter and the next one may be filled with some low temperature liquid, e.g. liquid nitrogen. The space to the next outer tube is again evacuated. In each instance one needs some kind of spacer construction between respective two concentric tubes facing each other directly in radial direction. A similar tube system can also be used as conduit for other low temperature fluids.
All these systems are economical only if the heat losses through conductor connection and radiation are very small. In all these systems one needs spacers which limit the heat flow across the space between the respective two tubes. See also, e.g. U.S. Pat. Nos. 3,826,286; 3,791,416. Particularly, the spacers as per the two earlier mentioned patents are very satisfactory, nevertheless, it is of great interest to further improve spacers for still higher impedance against flow of heat.